Ferroelectric materials, such as BaTiO.sub.3 and SrTiO.sub.3 are known to possess a low symmetry that allows them to be polarized by an electric field, as shown in FIG. 1a. As used herein, the term "ferroelectric material" refers to any material in which polarity can be permanently set by means of application of an electric field, and encompasses the specific materials enumerated above and any other materials having the described properties.
The polarization of ferroelectrics increases as the voltage across the film is increased. When the electric field passes a saturation-polarization threshold, the film remains permanently polarized in that direction, and will decay to a fixed remnant polarization some time after the voltage is removed. This phenomenon is shown in FIG. 1b.
Threshold voltage depends on the thickness of the film. For a typical 0.2 micron thin film, 3-5 Volts is quite ample to effect a change in polarization in either direction for most ferroelectric films.
FIG. 2 shows that there is a relationship between physical pressure on a polarized film 22 (where the direction of polarization is indicated by arrows 20), and induced voltage. When a polarized film is compressed, for example by a distance .DELTA.Z as shown in FIG. 2, a small displacement current through the film is generated in the compressed film. This results in charge accumulating at the surfaces of the film (e.g., on metal plates) which is the manifestation of the voltage induced by the applied pressure. Whether the voltage is positive or negative depends on the polarity of the film.
This mechanism also works in reverse. Particularly, if a voltage is applied across a polarized film, the film expands or contracts depending on the magnitude of the voltage, and whether the voltage is positive or negative with respect to polarization. This effect is characterized by the ferroelectric's piezoelectric coefficient, which is in the range of 200 pC/N for the typical kinds of ferroelectric films in use today.